Photochromic 2-( N -acyl- N -arylaminomethylene)benzo[ b ]thiophene-3(2 H )-ones with fluorescent labels and/or crown-ether receptors

Novel N -acylaminovinyl ketones of the benzo[ b ]thiophene series (AAB) capable of molecular switching of fluorescence or complexation with metal ions have been synthesized and their photochromic behavior studied using UV-Vis absorption/emission and 1 H-/ 7 Li-NMR spectroscopy. Under UV irradiation the AAB compounds 2a–c containing fluorescent labels as their N -aryl substituents undergo Z/E - photo-isomerization and N → O acyl rearrangement resulting in the formation of O -acyl isomers 3a–c . The photo-switching of fluorescence has been found in the 4-fluorenonyl derivative 2a . The benzo-15-crown-5-containing AAB 2d,e are also susceptible to the same type of photo-initiated rearrangement. The corresponding O -acyl isomers 3d–e represent a group of chemosensors which readily form stable complexes with alkali-and alkali-earth-metal cations. The AAB 2d exhibits specific selectivity for Li + ion in the presence of Na + , K + and Cs + .


Introduction
The photo-initiated reversible N→O acyl rearrangement of 2-(N-acyl-N-arylaminomethylene)benzo[b]thiophene-3(2H)-ones (AAB) has been found to be an efficient mechanism for abiotic solar energy storage. 1 Due to the presence of fluorescent and ligating centers in their molecules the AAB compounds may have a potential for use as molecular switches and photochromic fluorescent sensors for metal cations. 2,3In order to explore this possibility [4][5][6] we have prepared from aminovinyl ketones (AVK) 1 and investigated the properties of two groups of novel AAB derivatives: (i), compounds with fluorescent N-aryl substituents 2a-c, and (ii), benzo-15-crown-5-containing compounds 2d,e.The present paper reports on the synthesis, photochemical and fluorescent properties of these compounds, and their complexes with various metal cations.

Photochemistry
The ground-state structure of AAB 2a-c corresponds to the N-acyl form: their IR spectra are characterized by vibration frequencies related to the amide (1700-1720 cm -1 ) and exocyclic thiophene carbonyl groups (1660-1680 cm -1 ).Compounds 2a-c possess the Zconfiguration about the C=С bond, as indicated by the characteristic low-field resonance signals of the methine protons at 8.82-8.90ppm.These values were previously attributed to this form of AAB compounds, based on the 1 H NMR data and X-ray structure determinations, whereas the signals of the methine protons of the Eisomers were found at 5.90 ppm. 1,7Irradiation of acetonitrile solutions of AAB 2a-c (Table 1) with 436 nm filtered light results in Z/Eisomerization about the C=С bond followed by the thermal N→O acyl migration that affords stable O-acyl isomers 3a-c (Scheme 2).Long-wave absorption bands of the O-acyl compounds 3a-c are found in the spectral region 360-370 nm.Their IR spectra exhibit ester carbonyl group bands at 1780-1785 cm -1 (in CCl 4 ).Acidification of the solutions by adding small amounts of trifluoroacetic acid completely shifted the equilibrium to the initial form Z-2a-c.The cycle of 2 3 transformations can be produced repeatedly.The N-acyl AAB compounds 2b,c exhibit weak fluorescence with λ max = 468-475 nm.Its intensity is decreases to zero during the photo-initiated N→O acyl migration, in parallel with reduction of the content of the N-acyl form in solution.The only exception from this mechanism is given by the fluorenonyl derivative AAB 2a which did not show any fluorescence at room temperature.In contrast, the photo-induced O-acyl isomer 3a shows a relatively low intensity, but distinct, fluorescence (λ max = 545 nm, φ = 0.03) when excited with 350 nm light (Figure 1).To the best of our knowledge, this is the first example of a fluorescent benzenoid Oacyl form of type 3 compounds.As usual, these compounds do not exhibit luminescent properties, owing to the occurrence of fast intersystem crossing processes. 8n the solid state and in solution, crown-derivatized AAB compounds 2d,e exist in the form of the Z-isomer.This conclusion is validated by the IR spectra displaying absorption bands of the amide (ν C=O = 1660-1710 cm -1 ) and exocyclic thiophene (ν C=O = 1710 cm -1 ) carbonyl groups, the 1 H NMR low-field shift of the methine proton signals (8.66-8.90ppm), and the UV-Vis absorption spectra (Table 1).Irradiation of solutions of the AAB compounds 2d,e (λ irr = 436 nm) causes N→O acyl rearrangement to give the O-acyl isomers 3d,e (Scheme 3).The structures of the initial form Z-2d, the photo-isomer 3d and the corresponding lithium complexes 2d .Li + , 3d .Li + were proved by the 1 H-and 7 Li-NMR spectral data (Table 2).The crown-ether methylene proton signals exhibit low-field shifts of 0.06-0.10ppm in the complexes 2d .Li + , 3d .Li + as compared with the isomers 2d,3d respectively.The protons more distant from the crown moiety do not measurably respond to the presence of Li + in the crown-ether ring.In the 7 Li NMR spectrum of the compound 3d .Li + in acetonitrile the lithium signal is shifted by 0.78 ppm to low field with respect to LiI.These ionic equilibria are specifically reflected in the UV-Vis absorption spectra of these compounds.No significant changes are observed in the spectra of the AAB compounds 2d,e at 425-430 nm in the presence of alkali-and alkali-earth-metal (MI and M(ClO 4 ) 2 ) ions in acetonitrile, isopropanol or DMSO solutions.On the other hand, the photo-initiated O-acyl isomers 3d,e, absorbing at 350-360 nm, readily form complexes 3d,e .M n+ as evidenced by a substantial increase in the extinction of those bands (∆ε) and their blue shifts (∆λ) (Table 3, Figure 2).Under UV-Vis irradiation the compounds 2d,e .M n+ also undergo N→O acyl shift with formation of the same complexes 3d,e .M n+ .The quantum yields of this rearrangement correlate with the alkali metal ion radii (Table 3).The photo-isomer 3d exhibits unique selectivity with respect to Li + cation in acetonitrile.For the lithium complex 3d .Li + , the characteristic spectral parameters (∆ε, ∆λ) are not affected by the presence in the solution of other alkali metal ions

Conclusions
Novel 2-(N-acyl-N-arylaminomethylene)benzo[b]thiophene-3(2H)-ones with fluorescent labels and/or crown-ether receptors susceptible to photo-initiated reversible Z/Ephoto-isomerization followed by thermal N→O acyl migration represent a new type of molecular switches of fluorescence and chemosensors for metal ions.

Experimental Section
General Procedures. 1 H-and 7 Li-NMR spectra were recorded on a Varian Unity 300 (300 MHz) spectrometer with internal standard HMDS.IR spectra in Nujol were measured using a Specord 75IR spectrometer.UV-Vis absorption spectra in CH 3 CN were obtained with a Specord M-40 spectrophotometer.Irradiation of solutions was carried out by filtered light of a highpressure mercury lamp DRSh (250 W) supplied with a set of glass filters.Potassium ferrioxalate was used as an actinometer for the quantum yield calculations. 9 Fluorescence spectra in CH 3 CN were measured on a Hitachi 650-60 spectrofluorimeter.The fluorescence quantum yield of AAB 3a was determined using fluorenone as the standard.

Scheme 2 Table 1 .
Spectral and luminescent characteristics of AAB 2 in acetonitrile solutions

Table 3 .
Spectral changes in long-wave absorption band of complexes